Piezoelectric crystal element and method of fabricating same



April 13, 1948. c. K. GRAVLEY 2,439,456 PIEZOELECTRIC CRYSTAL ELEMENTAND METHOD OF FABRICATING SAME Filed Oct. 20, 1944 JNVENTOR. CHARLES l1.GR/WLEY ATTORNEY Patented Apr. 13, 1948 PIEZOELECTRIC CRYSTAL ELEMENTAND METHOD OF mnmcarmc SAME Charles K. Gravley, Cleveland Heights, Ohio,as-

signor to The Brush Development Company, Cleveland, Ohio, a corporationof Ohio Application October 20, 1944, Serial No. 559,524

My invention pertains to a method of fabricating a moisture-proofedpiezoelectric crystal element, and to a new and novel piezoelectriccrystal element.

An object of my invention is to provide a piezoelectric crystal elementwhich is more moisture resistant than hitherto known piezoelectriccrystal elements of comparable type.

Another object of my invention is to provide a method of constructing apiezoelectric crystal element so that it is more moisture resistant.

A further object of my invention is to provide a method of evenlydistributing a liquid coating material on a piezolectric crystalelement.

It is also an object of my invention to provide a method of fabricatinga piezoelectric crystal element having a moisture resistant coat ofsubstantially even thickness throughout its flat surface areas and atits edges and corners.

Another object of my invention is toprovide a piezoelectric crystalelement whose' corner and edge areas are substantially as moistureresistant as its face areas.

Another object of my invention is to so shape a piezoelectric crystalelement that the maximum effectiveness in waterproofing may be obtainedfrom any given waterproofing material.

Other objects and a fuller understanding of my invention may be had byreferring to the following description and claims read in conjunctionwith the drawings in which:

Figure 1 is a cross-sectional view of a mom-- electric plate which hasnot been moistureproofed according to my invention.

Figure 2 is a cross-sectional view of a piezoelectric crystal elementwhich has been moisture-proofed according to my invention.

Figure 3 is a cross-sectional view of a multiplate flexing typepiezoelectric element fabricated according to my invention.

Figure 4 is a diagrammatic illustration of a 15 Claims. (Cl. 171-327)causing it to pull away from sharp edges and corners. In waterproofingpiezolectric crystals, which are usually rectangular, square, ortrapezoidal, this "pulling away" of the coating material from thecorners results in weak spots in the coating through which moisturepasses to the detriment of the crystal element.

In my invention I provide a practical method for rounding the edges andcorners of a piezoelectric crystal plate sufliciently to cause theliquid coating material to be evenly distributed on the crystal elementso that when the coating dries the resulting moisture-resistant coat isof substantially even thickness throughout the area of the crystalelement. Thus there is no weak spot in the moisture-resistant coat atthe edges and corners of the element.

Figure 1 is a cross-sectional view of a piezoelectric crystal element 10whose corners i2 have not been rounded. A coat of water-proofingmaterial l3 has been applied and the figure shows how the coatingmaterial l3, due to surface tension, tends to pull away from the cornersleaving the comers l2 bare or only very slightly covered by themoisture-proofing material [3. My Patent No. 2,359,476 shows a deviceand method for reducing the extent to which the coating pulls away fromthe corners and edges.

When the crystal element I0 is subjected to an atmosphere having highhumidity the moisture will first break through at the comers and edgesand is then apt to travel under the moisture-proof coating thusspreading over a relativelylarge area. of the crystal element andleading to failure in or serious reduction of its piezoelectric action.

Piezoelectric crystal elements almost always have electrodes associatedwith them, such as the electrode [4, and it has been found that ifmoisture once gets under the electrode it is apt to spread rapidly. Alsoif moisture spreads on 'the face of the crystal element between the twoelectrodes H-H', a shunt circuit is established which materiallyadversely affects the operation of the crsytal element.

Figure 2 illustrates a piezoelectric crystal element I5 whose comers ithave been rounded by my process. Subsequent to the rounding operationthe crystal element I! has been coated with a liquid coating materialsuch as shellac, chlorinated rubber. or the like, and it shows how theliquid coating material l3 dries to form a moisture-resistant film ofsubstantially uniform thickness throughout the face. edge and cornerareas of the crystal element. Thus the element shown in Figure 2 is asmoisture resistant at its corners' and edges as it is at its face areas.

Figure 3 illustrates a multiplate flexing element of the type disclosedin Sawyers Patents Re. 20,213 and Re. 20,680, moisture-proofed inaccordance with my invention. The corners and edges 38 on the upper orouter face of the plate 31 are rounded, but the corners and edges on thelower or inner face are not rounded. The corners and edges 40 on thelower or outer face of the plate 39 are rounded, but the corners andedges on the upper or inner face are not. I

A preferred method of fabricating a multiplate flexing element comprisesthe following steps: select two plates of crystalline material of propersize and orientation, as has been described in the Sawyer patents;machine one face of each plate until it is smooth, as has been describedin Wil- 'iiams Patent No. 2,414,574, or in Sawyer et al.

2,112,636; connect an electrode 36, 36' to each of said machined facesusing the teachings of Williams 2,106,143, or Sawyer 1.995257 and1,994,- 487; cement the two plates together face-to-face with theelectrodes inbetween, and if desired, with a lead extension in contactwith said inner electrodes and extending out away from said element; oneof the-outer faces is machined to provide a smooth surface and to reduceone of the connected plates to its proper thickness, and the edges andcorners of the plate are rounded; the other outer face of the multiplateassembly is then machined and its edges and corners are rounded; outerelectrodes 35, 35' are then connected to the outer faces, Leads may beconnected to the electrodes 35, 35 if desired and the assembly is thendipped in or sprayed with liquid material which, upon drying, forms amoisture-resistant coating 13 of uniform thickness around the element.

Figure 4 illustrates the preferred method of rounding the corners andedges it of a piezoelectric crystal element. A circular work-supportingtable I is keyed to a vertical spindle 3 which receives relatively slowrotary motion from a prime mover which is not illustrated. Suitablebearings are provided for the spindle and the table, if desired, may besupported at its periphery upon a fixed circular track of conventionaltype. The table is provided with a large number of small channels 5 thatterminate in openings in the upper and lower faces thereof. A rotarymilling cutter l is mounted to extend inwardly over the upper face ofthe table from the periphery toward the spindle, the axis of rotation ofthe cutter being parallel to the plane of rotation of the table. Aplurality of wire brushes 20 and 2| are mounted to extend over the tableI and are driven by means such as a motor 2'2. Means (not shown) areprovided for raising and lowering-the cutter and the brushes, eithertogether or independently, with respect to the surface of the table andthe means may be set to hold the cutter and the brushes a spaceddistance above the table.

The upper surface of the table I carries a. masking disk 9 having aplurality of spaced crystal receiving windows H therein. The spindle 3has a threaded extension which extends through a central aperture in themasking disk 9 and a nut 25 having a wide laterally extending flange 26is threaded on the spindle and so holds the disk 9 rigidly yet removablyin fixed position that it revolves with the table. The windows ii in thedisk 9 expose only the openings of the channels extending through thetable directly beneath them; the remaining channels are Ill blocked oflby the imperforate portions of the disk. The windows ii are onlyslightly larger than the crystal section which is to be put into it thusfacilitating loading and unloading of the crystal. Should it be desiredto operate upon crystal sections of a different size or shape, adifferent masking disk QImay be placed on the table l. i

In order that atmospheric pressure /may (be utilized automatically forholding a crystal section in place upon the table during the millingoperation, the open end of an intake pipe 25 leading to an exhaust pump(not shown) is positioned beneath the table directly below the millingcutter and the gap between the lower surface of the moving table and theedge of the open end is made air-tight through utilization of packingmeans such as a resilient gasket disposed around the pipe and maintainedin engagement with the underneath side of the table I. A more detaileddescription of this gasketmay be had by referring to Alfred L. W.Williams Patent No. 2,414,574.

Similar exhaust pipes 26 and 27 and resilient gaskets are provided forexhausting the air under those crystal elements which are directlybeneath the brushes 2!] and 2!. Obviously means may be provided wherebya singl pipe exhausts the air at all three work stations.

It is preferable that the milling cutter i rotate in a direction tooppose the movement of a crystal section moving past it. One of the wirebrushes, for example, brush 20 should also oppose the movement of thecrystal section past it but the other brush 2! should rotate in adirection to aid the movement of a crystal section past it. The worktable 5 rotates in a direction indicated by the arrow 28 such that eachcrystal element is milled prior to being brushed. After passing themilling cutter 1 each crystal element is brushed for a period of'time ofabout to 1 second in order to round two of its edges and it then passesto the brush 2| where it is brushed in the reverse direction for aboutto 1 second in order to round two more of its edges. After being brushedthe second time the crystal elements are removed from the work table ata point where atmospheric pressure does not tend to hold them down andif they are not to form part of a multiplate flexing element, may go toa second similar work table where the opposite face is milled and theother 4 edges are brushed.

By suitable choice of the brushes 20, 2| and by suitably correlating therate of rotation of the brush and the time period during which thecrystal is operated on by the brush, the edges and corners may berounded without detrimentally affecting the face area of the elements.Factors which must be taken into consideration when choosing a suitablebrush include the material from which the bristles are made, thediameter of the brush, the compactness of the bristles of the brush, andthe diameters of the individual bristles in the brush. Thus it isvirtually impossible to here define all brushes which will workespecially when the best type of brush will also depend somewhat uponthe size and type of crystal element whose corners are to be rounded. Asa practical example, I have found that on Osborn brush model No. 604-0is satisfactory for rounding the edges and corners of Rochelle saltcrystal elements. Two of these brushes may be positioned with respect tothe crystal elements as shown in Figure 3. Their elevation with respectto the crystal should be such that about a: to %4 of an inch of thebrush hits the crystal. This brush has steel wire bristles, 4 mils indiameter and about 1.75 inches long. The brush itself is 6 inches indiameter and it was rotated at 1,800 revolutions per minute. [have alsofound that if the bristles are of brass wire then thew may be 4 or 6mils in diameter and still give satisfactory performance. Care must beexercised that the brush does not rotate too rapidly as it has beenfound that the centrifugal force stiffens the Wires to an extent thatthey will scratch the face surface of the. crystal elements. The brush20 rotating in the direction shown by the arrow 29 opposes the motion ofthe crystal elements and rounds the two leading edges 3t, 32 of thecrystal. The brush 2| rotating in the direction of the arrow 30 to aidthe motion of the crystal elements rounds the trailing edges 33 and 34of the crystal. Thus after each crystal element passes the second brush2| its four top edges and corners are rounded. By a subsequent operationthe four bottom edges and corners may be rounded. I have found thatabout a m or a 3 inch radius is sufficient rounding to give good resultswhen water-proofing material such as shellac or chlorinated rubber isused for covering the crystal elements. Generally speaking the higherthe surface tension of the material to be used for covering the crystalelements the more the corners and edges should be rounded in order toassure good coverage.

This disclosure has been made by way of example, and many changes may bemade without departing from the spirit and scope of the invention asdefined by the claims.

I claim as my invention:

1. In the process of fabricating a piezoelectric crystal element, thesteps of rounding the edges and corners of said element, subsequentlycovering said element with a coating of hardenable liquid material, andcausing said liquid coating to become hard to form a moisture-proofcoating.

2. In the process of fabricating a platelike piezoelectric crystalelement, the steps of rounding all of the edges and corners of saidelement adjacent one of its major faces. substantially covering saidelement with a coating of hardenable liquid material, and causing saidliquid coating to become hard to form a moisture-proof coating, saidhard moisture-proof coat being substantially as thick at said edges andcorners as it is on the said major face of said element.

3. In the process of fabricating a piezoelectric crystal element, thesteps of causing said element to come into contact with a rotating brushto cause said brush to remove crystalline material at the edges andcorners of said element, and subsequently coating said element with athin layer of liquid hardenable waterproofing material.

4. In the process of fabricating a plate-like piezoelectric crystalelement, the steps of causing said element to come into contact with arotating brush to cause said brush to remove crystalline material toround all of the edges and corners of said element adjacent one of itsmajor faces, and subsequently coating said element with a thin layer of.liquid hardenable waterproofing material, said moistureproof layer uponbecoming hard being substantially as thick at said edges and corners asit is on the faces of said element.

5. The process at set forth in claim 4, further characterized in this:that said brush is a wire brush.

6. The process as set forth inclaim 4, further characterized in this:that said brush is a wire brush comprised of brass wires 4-6 mils indiameter.

7. The process as set forth in claim 4, further characterized in this:that said brush is a wire brush comprised of steel wires 4 mils indiameter.

8. The process as set forth in claim 4, further characterized in this:that said brush is about 6 inches in diameter and rotates at about 1,800r volutions per minute.

9. I'he process as set forth in claim 4, further characterized in this:that only about 3% to %4 of an inch of the ends of the brush bristlescome in contact with said crystal element.

10. The process as set forth in claim 4, further characterized in this:that the direction of rotation of said brush is at an angle with respectto the directions of the edges of said crystal element.

11. The process as set forth in claim 4, further characterized in this:that said brush is in contact with said crystal element for about onesecond.

12. The process as set forth in claim 4, further characterized in this:that said brush is about six inches in diameter and is comprised of wirebristles about 5 mils in diameter, said brush rotating at about 1,800revolutions per minute and havirg about as of an inch of the ends of thebristles in contact with the crystal element for about one second.

13. As an article of manufacture, a piezoelec tric crystal elementcovered with a thin layer of substantially waterproof material, saidcrystal element having all of its edges and corners rounded.

14. As an artcle of manufacture, a piezoelectric crystal element coveredwith a thin layer of substantially waterproof material, said crystalelement having all of its edges and corners rounded. and said layer ofsubstantially waterproof material being of substantially uniformthickness at all points on said element.

15. An article of manufacture as set forth in claim 14, furthercharacterized in this: that the said edges of said crystal element arerounded on a radius of about 41 of an inch.

CHARLES K. GRAVLEY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 300,711 Ingersoll June 17, 18841,950,372 Cross Mar. 6, 1934 1,964,174 Raiche June 26, 1934 2,123,227Bieling July 12, 1938 2,405,425 Higgins Aug. 6, 1946 2,359,476 GravleyOct. 3, 1944

